Method and apparatus for drawing crystals from a melt



May 8, 1962 B. OKKERSE METHOD AND APPARATUS FOR DRAWING CRYSTALS FROM AMELT Filed April 6, 1960 INVENTOR BOUDEWIJN OKKERSE AGENT United StatesPatent 3,033,660 METHOD AND APPARATUS FOR DRAWING CRYSTALS FROM A MELTBoudewijn Okkerse, Eindhoven, Netherlands, assignor to North AmericanPhilips Company, Inc., New York,

N.Y., a corporation of Delaware Filed Apr. 6, 1960, Ser. No. 20,360Claims priority, application Netherlands May 5, 1959 11 Claims. (Cl.23301) This invention relates to a method of drawing crystals from amelt. Such a method is commonly employed for manufacturingmonocrystalline bodies from semi-conductor materials, such as germanium,silicon and intermetallic compounds. These bodies have to satisfy therequirement of homogeneity, that is to say, that their crystal latticeexhibits few dislocations and that the physical properties, for examplethe specific resistance, are the same at any point of the crystal.

In a known method, the melt is produced in a cylindrical crucible havinga flat or concave bottom; then a socalled seed crystal is brought intocontact with the melt and subsequently slowly drawn upwards so thatmaterial crystallizes out and grows to the crystal lattice of the seed.

The homogeneity of the resulting body may show deviations; it maydeviate in the direction of drawing, that is to say, in the verticaldirection, and at right angles thereto. The homogeneity in the formerdirection is influenced inter alia by the composition of the melt and byvariations in the drawing rate. The factors which can influence thetransverse homogeneity, however, were not exactly known.

It has already been suggested to rotate the seed during the drawingprocess in order to avoid dislocations and other defects in the crystal.It was found that this measure materially reduces the occurrence of suchdefects but does not completely eliminate them, and it is an object ofthe present invention to create better conditions for homogeneity of thecrystal.

The present invention is based on the recognition that defects in thecrystal are due to temperature fluctuations in the melt, in particularin the part of the melt immediately under the crystal.

Uncontrollable convection currents and/or eddies must be considered asan important cause of the occurrence of fluctuations in the melt.

It is a particular object of the invention to create more favourableconditions in the melt under the crystal. According to the invention,the crucible has a part situated above the bottom, which parthereinafter will be referred to as a projection and the diameter ofwhich is at least equal to that of the drawn crystal.

Due to the provision of the projection the convection currents encountera greater flow resistance in the shallow part of the melt over thisprojection than in the deeper part of the melt, which hereinafter willbe referred to as an annular channel, whereas the currents in thisannular channel enhance the uniformity of the melt both with respect tothe temperature and to the composition thereof. In addition, the crystalis rotated about its axis in the usual manner, the comparatively thinlayer of the melt under the crystal thus being thoroughly stirred.

. Preferably the projection is cylindrical; its upper surface ispreferably flat or substantially flat.

The inner diameter of the crucible is preferably at least twice thediameter of the projection. Thus, the annular channel has a width suchthat the flow resistance is low enough both for horizotnal and forvertical convection currents. Hence a disturbing influence of the heatexchange between the lateral wall of the crucible and the melt will behardly perceptible near the growing crystal.

3,033,660 Patented May 8, 1962 The upper surface of the projectionpreferably lies at a level with respect to the bottom of the crucible ofat least one third of the diameter of the projection. With such a levela possible horizontal temperature gradient in the bottom of the cruciblewill be scarcely perceptible at the upper surface of the projection.Hence, the temperature at the upper surface will depend to a higherdegree upon the heat exchange between the projection and the surroundingmelt, in which there are no large temperature variations.

During the drawing process, the spacing between the crystal and theprojection is preferably maintained con stant.

This may be effected by making the projection vertically movable withrespect to the crucible and gradually lowering it during the drawing ofthe crystal according as the level of the melt falls. The projection maybe shaped as a disc arranged on a shank which is passed through anaperture in the bottom and by means of which the position of theprojection can be controlled from with out. The clearance between thewall of the aperture and the shank must be so slight that the upwardpressure due to the surface tension exceeds the hydrostatic pressure ofthe melt. The upward pressure can be calculated in grams per sq. cm. bymeans of the following formula:

see

where 7 is the surface tension in dynes/cm., g the acceleration ofgravity in cm./sec. and R and R are radii of curvature of the surface ofthe liquid in two directions at right angles to one another in cm. For aclearance of 0.2 mm., with molten germanium contained in a graphitecrucible, the surface tension is to be assumed to be 500 dynes/cm., R Acm. and R =infinity. If it is further assumed that g=l000 cm./sec. theupward pres-, sure is found to be 50 g./sq. cm.

The hydrostatic pressure of the germanium melt must not exceed thisupward pressure and consequently the level of the melt must lie at mostslightly more than 8 cm. above the bottom, and this is generallysufficient in practice.

The constant spacing between the crystal and the projection mayalternatively be maintained by supplying melt to the crucible during thedrawing of the crystal so that the level of the melt in the crucible isconstant. For this purpose, the crucible preferably, by means of a duct,communicates with a container the volume of which can be varied.Preferably this container comprises a cylinder and piston arranged underthe crucible. Between the piston and the cylinder a small clearance isagain admissible provided that the upward pressure due to the surfacetension again exceeds the hydrostatic pressure of the melt filling thecontainer.

According to a further preferred embodiment, the level of the melt ismaintained at a level of at most 6 mm. above the upper surface of theprojection.

The invention will now be described more fully with reference to someembodiments illustrated by the accom= panying drawing. In the figures ofthis drawing, apparatus in accordance with the invention is shown partlyin cross section and partly in perspective.

FIGURE 1 shows an apparatus in which the projection is verticallymovable.

FIGURE 2 shows an apparatus in which the pro'jec-l tion has a fixedposition relative to a crucible communicating with a container. I

Example I a mm. An aperture 3" is centrally of the bottom 2 of thecrucible; The projection" comprises a graphite disc 4' having a diameterof 18 mm. and a height of 6 mm. This disc is arranged on a verticalshank 5, which passes through the aperture 3 with a clearance of 0.2 mm.and is capable of free vertical movement through this aperhim, so thatthe position ofthe disc 4 can readily be adjusted from without.

The crucible is arranged on a hollow cylindrical support 6 and issurrounded by a high-frequency coil 7. The crucible contains moltengermanium 9 which cannot flow away through the aperture 3 since theupward pressure produced by the surface tension exceeds the hydrostaticpressure. The high-frequency coil 7 is energized so that heat isproduced in the lateral wall 8 of the crucible. This heat is transferredto the melt 9 so that convection currents are produced therein.

The disc 4 is adjustedso that its upper side It) lies 4 mm. below thelevel of the melt. Thus, the convection currents will be kept away fromthe melt over the disc 4 and will travel along an annular channel 11.

A rod-shaped germanium crystal12 having a diameter of 15 mm. is incontact with the melt over the disc 4' arid is drawn upwards at a rateof 1 mm. per minute so as to grow. While being drawn up, the crystal maybe rotated about its vertical axis. During the drawing process, thelevel of the melt in the crucible falls. Now the disc 4 is graduallylowered so that the ditference in height between the level of the meltand the upper surface of the disc is maintained at 4 mm. and theldiquilcll layer under the crystal is maintained at a constant ept g Thecurrent strength in the coil is controlled so that the diameter of thegrowing crystal is maintained at mm. so'that the crystal does notproject beyond the disc 4 horizontally.

Example II In this example use is made of an apparatus shown in FIGURE2. In this figure, reference number 29 denotes a cylindrical graphitecrucible having an inner diameter of 50 mm., the bottom 21 of thiscrucible being provided with a central cylindrical projection 22 havinga diameter of mm; and a height of 10 mm. This projection has a flatupper surface 23.

The crucible 20 is supported by a graphite disc 24 having .a rim 25projecting on either side of the disc. The'rimmed disc is supported by ahollow graphite cylinder 26'closed at the .upper end. Owing to theprovision of the broad-rimmed disc two chambers 27 and 28 havinghorizontal boundary surfaces are produced between the bottom 21 and theupper surface of the cylinder 26.

A vertically movable piston 29 provided with a connecting rod 30 fitswithin the cylinder 26 with a clear anceof 0.1 mm. The cylinder 26 andthe piston 29 form a container 31 of variable volume. The container 31'is connected to the crucible 20 by a duct 32 which extends in partthrough a rod-shaped connecting member 33 interposed between the uppersurface of the cylinder 26 and the bottom 21 of the crucible and passingthrough an aperture 34 in the disc 24.

The assembly comprising the crucible 20, the disc 24 and the cylinder 26containing the piston is arranged on a hollow support 36 and surroundedby a high-frequency coil 35. m

In operation, .the container 31 and the duct 32 are completely filledand the crucible 20' is partly filled with molten germanium 38 Theconnecting rod of the piston 29 is adjusted so that the level of themelt 38 in the crucible is at a height of 3 mm. above the upper surface23 of the projection 22.

The high-frequency coil is' energized so that heat is produced in thewalls of the crucible 20 and the container 31' and also in the rim 25.Due to the presence of disc 24' provided with the rim 25 and thechambers 4- 27 and 28, additional heat is supplied to the bottom 21 ofthe crucible and to'the upper end of the container 31.

The convection currents produced in the melt 38 are restricted to anannular channel 39 surrounding the projection 22 so that the layer ofthe melt over the projection is not influenced by these currents. 7

By means of a drawing mechanism (not shown) a germanium crystal 40 isdrawn from the melt at a point directly over the projection 22 at a rateof 1 mm. per minute so as to grow gradually. By controlling thehighfrequency current in the coil the diameter of the growing crystal ismaintained at 19 mm. so that it does not project horizontally beyond theprojection 22. By rotation of the crystal, the part of the melt over theprojection can be thoroughly stirred.

By the gradual growth of the crystal, melt is continuously beingwithdraum from the crucible. By simultaneously raising the piston 29,however, melt is supplied to the crucible from the container 31 throughthe duct 32. The: speed. at which the piston travels is controlled sothat the level of the melt 38 in the crucible is maintained at a heightof 3 mm; above the upper surface 23 of the projection 22.

What is claimed is:

l. A method of drawing crystals from a melt contained in a cruciblehaving bottom and side wall portions and a projection extending upwardlyfrom the bottom wall within and spaced from the said side walls andwhose upper extremity possesses a given diameter, comprising the stepsof maintaining the melt within the crucible at a level above the upperextremity of the projection, and drawing a crystal upwards from a regionof the melt lying over the said extremity.

2. A method of drawing crystals from a semicondu tive melt contained ina crucible having bottom and side Wall portions and a projectionextending upwardly from the bottom wall within and spaced from the saidside walls and whose upper extremity possesses a given diameter,comprising the steps of maintaining the melt within the crucible at alevel just above the upper extremity of the projection, and drawing agrowing crystal having a diameter not greater than said given diameterupwards from a region of the melt lying only over the said extremity. 4

3. A method as set forth in claim 2 including the step of maintainingsubstantially constant the meltlevel above the said upper extremity bylowering the projection while the crystal is drawn upwards.

4. A method as set forth in claim 2 including the step of maintainingsubstantially constant the melt level above the said upper extremity byintroducing melt into the crucible while the crystal is drawn upwards.

5. A method of drawing crystals from a semiconductive melt contained ina crucible having bottom and side wall portions and a projectionextending upwardly from the bottom wall within and spaced from the saidside walls and whose upper extremity is cylindrical possessing a givendiameter and has a flat upper surface, comprising the steps ofmaintaining the melt within the crucible at a level not more than 6 mm.above the upper surface of the extremity of the projection, and drawinga growing crystal having a diameter not greater than said given diameterupwards from a region of the melt lying only over the said extremity.

6. Apparatus for drawing crystals comprising a crucible having bottomand side wall portions and a projection extending upwardly from thebottom wall within and spaced from the said side walls and whose uppereX- tremity possesses a given diameter, a melt of semiconductivematerial within said crucible, means for maintaining the melt at a leveljust above the upper extremity of the projection, and means for drawinga crystal upwards from a region of the melt lying only over the saidextremity.

7. Apparatus as set forth in claim 6, wherein the projection isvertically movable, and means are provided for lowering the projectionwhile the crystal is drawn upwards to maintatin the melt level justabove the said extremity.

8. Apparatus as set forth in claim 6, wherein means are provided forfeeding melt into the crucible to maintain the melt level.

9. Apparatus for drawing crystals comprising a crucible having bottomand side Wall portions and a projection extending upwardly from thebottom wall Within and spaced from the said side walls and Whose upperextremity possesses a substantially flat upper surface and iscylindrical with a given diameter, a melt of semiconductive materialwithin said crucible, means for maintaining the melt at a level justabove the upper extremity of the projection, and means for drawing agrowing crystal having a diameter not greater than the said givendiameter upwards from a region of the melt lying only over the saidextremity.

10. Apparatus as set forth in claim 9, wherein the spacing between thecrucible side walls is at least twice the said given diameter, and theflat upper surface of the said extremity is spaced from the bottom wallat a distance at least equal to one-third the said given diameter.

11. Apparatus as set forth in claim 9, wherein the melt level ismaintained at a distance not more than 6 mm. above the upper surface ofthe said extremity, and said distance is maintained substantiallyconstant.

References Cited in the file of this patent UNITED STATES PATENTS1,552,884 Schnurpfeil Apr. 8, 1925 2,793,103 Emeis May 21, 1957 FOREIGNPATENTS 754,767 Great Britain Aug. 15, 1956

1. A METHOD OF DRAWING CRYSTALS FROM A MELT CONTAINED IN A CRUCIBLEHAVING BOTTOM AND SIDE WALL PORTIONS AND A PROJECTION EXTENDING UPWARDLYFROM THE BOTTOM WALL WITHIN AND SPACED FROM THE SAID SIDE WALLS ANDWHOSE UPPER EXTREMITY POSSESSES A GIVEN DIAMETER, COMPRISING THE STEPSOF MAINTAINING THE MELT WITHIN THE CRUCIBLE AT A LEVEL ABOVE THE UPPEREXTREMITY OF THE PROJECTION, AND DRAWING A CRYSTAL UPWARDS FROM A REGIONOF THE MELT LYING OVER THE SAID EXTREMITY.